In electrical machines, in particular in rotating electrical machines, such as for example motors or generators, what are known as stator winding bars are used in order to homogenize the current density within the electrical machine and to reduce induced eddy currents as far as possible. The rotating machines are constructed for this purpose from a multiplicity of partial conductors or strands. The strands are mutually insulated by means of an appropriate partial conductor insulation, as is known to the expert. Roebel-stranding of the partial conductors is furthermore a common procedure in order to minimize losses caused by induced eddy currents. In the manufacture of large electrical machines, which also can be referred to as rotating machines, it is necessary to bond the strands in order to achieve the desired insulation of the strands and thereby their electrical insulation. For this purpose the partial conductors are constructed as what is known as stator winding bars; these contain a desired degree of Roebel-stranding, and are bent if necessary. Like the bending, the desired Roebel-stranding causes mechanical stress, which can have the result that the strand insulation is damaged, and that electrical contacts can occur between individual instances of the strands of a stator winding bar. These unwanted electrical connections between strands impair the electrical insulation of the strands, and when the machine, which may be a rotating (three-phase) motor or a generator, is operating they lead to a risk of premature failure. The service life and/or the capacity of the machine is significantly impaired as a result. It is therefore of interest to detect corresponding possible damage to the strand insulation as early as the manufacturing stage. It was usual in the past to check the strands of a stator bar before the stator winding bar or the strand stator bar was consolidated at its two ends, i.e. electrically connected to the other parts of the machine. For this purpose it was usual to use a suitable measuring instrument such as a multimeter to check the stator bar or the stator winding bar for possible inter-strand short-circuits, that is to say an electrical connection between individual instances of the strands, which indicates damage to the electrical insulation between individual strands of the multiplicity of strands within the stator winding bar. It was in the past necessary to use a multimeter to check each strand against the other strands for an electrical connection, particularly a low-resistance connection.
This procedure in the prior art was very time-consuming and expensive since for one stator winding bar, which is constructed, for example, from 50 strands that are correspondingly Roebel-stranded together, it is necessary to check each strand for insulation with respect to the other strands within the stator winding bar. This means that a total of 1225 combinations of strands must be checked. 1225 measuring steps are thus necessary for a complete check. In addition to this, in the prior art the appropriate measurement using the multimeter, as described above, can only take place as long as the strands that are going to be brought together to form the stator winding bar have not yet been consolidated. That means that these measurements must already be carried out before the application of the appropriate insulation, that is before installation of the Roebel bars into the laminated stator core, since a measurement is no longer possible at a later time. As a particular consequence, early damage that occurs, for example, during transport, which only leads to low-resistance connections between the strands when subjected to additional mechanical stress, such as the pressing after installation of the bars, or Roebel bars, into the laminated core, is not detected. This means that in the prior art it could happen that the time-consuming measurement has not indicated any damage to the insulation between the strands, although said damage is present after installation has been completed.
US 2011/304340 A1 discloses a method for testing an electrical conductor for an anomaly. U.S. Pat. No. 7,906,973 B1 describes a network device with a control module and a cable test module. A method for insulating an electrical circuit in an aircraft is described in US 2004/100272 A1. EP 2 202 529 A2 discloses a method and an apparatus for cable inspection. DE 10 2013 202717 A2 describes a modular cable harness and a modular cable harness test apparatus.